1. The Field of the Invention
The present invention relates to disk drives by which data can be written to or read from magnetic storage media. More specifically, the present invention relates to disk drives having multiple recording heads that are capable of performing parallel read and write operations, which increases the read and write data rates.
2. The Relevant Technology
During recent years, there has been a steady improvement in the volume of data that can be stored on magnetic storage media, such as hard disks drives used in computers. As the areal density of the storage media associated with disk drives has increased, storage capacity per unit cost has fallen dramatically, which has enabled individual users and enterprises to radically change the way in which data is recorded and stored. Indeed, the ability to store large volumes of data inexpensively has been a driving factor in the information technology revolution during recent decades.
One of the key issues plaguing the drive industry over the past several years is that the growth in areal density of the magnetic storage media has not been matched by a corresponding growth in sustained data rate. During recent years, the areal density growth has been about 100% per year, whereas the sustained data rate associated with disk drives has grown at only about 30% per year. This is illustrated in FIG. 1, which graphs the storage capacity, the peak data rate, the sustained data rate, and the inverse of the access time for high-performance disk drives in recent years, as well as projections for the coming years that have been made by experts in the industry in view of the expectation of incremental advancements based on current technology. Storage capacity is generally expressed in terms of bytes of data that can be stored per unit area of disk. The peak data rate is the maximum data rate is expressed in terms of bits per unit time. The peak data rate typically relates to the peak write data rate that can be reached for a short period of time, and is generally achieved using buffering as described below. The sustained data rate is measured in bits per unit time and is defined as either the write or read data rate that can be achieved for write or read operations having an arbitrarily long duration. The access time is defined as the time between the receipt of a read request and the time at which the requested data is under the recording head.
FIG. 2 illustrates a conventional hard disk drive 10, which includes a single disk 12 and a single head gimble assembly 14 with a macroactuator 16 and a slider, or recording head 18. In operation, a transducer positioned on the recording head 18 reads data that is magnetically encoded on the surface of the disk 12 or writes data to the surface of the disk. In order to access the appropriate sectors on the disk, the macroactuator 16 uses a closed-loop feedback or servo process to detect the position of the recording head 18 and adjust the position as needed. Other conventional disk drives have multiple recording heads and can include multiple disks in a disk stack. However, existing hard disk drives write and read data using one recording head at a time, on various surfaces within the disk stack, rather than using multiple recording heads simultaneously, due to several physical limitations. The inability to use more than one recording head 18 at a time is a factor that has significantly limited improvements in the data rates of disk drives, as shown in FIG. 1.
The maximum sustained data rate is limited by the linear bit density that can be sustained on a disk 12 and the disk rotational velocity. For disk drives that operate at 15,000 rpm, which is the highest speed currently available, and that have an inner disk diameter of about 3.3 inches and a linear density of 500 kilobits/inch, the maximum sustained data rate can be no greater than about 1 Gb/s. In addition, eddy currents in the soft magnetic material used in the recording head 18 limit switching speeds so that data rates are typically no greater than 500 Mb/s. Other physical limitations to reading and writing at data rates of over about 500 Mb/s include thermal stability of media with very short bit lengths and reduced sense amplitude due to decreased shield-shield gap spacings on giant magnetoresistive (GMR) heads. It is expected that areal density will continue to improve during the coming years. However, it is likely that a large percentage of the gains to be made in areal density in the next few years will relate to increased track density, which would make the data rate problems even worse.
Because of the relatively small increase in the maximum available sustained data rate in individual disk drives, systems using redundant arrays of independent disks (RAIDs) have become widely used for enterprise data storage. Such RAID systems involve parallel formatting and use of existing high capacity disk drives, which allow the overall data rate for RAID systems to be significantly higher than rates for individual disk drives.
The striping and caching techniques that are used to sequence the timing of separate, multiple disk drives in a RAID that is capable of operating at data rates of five to ten times that of individual disk drives are quite complex. RAID systems operate at various levels, which relate to the degree to which the data is distributed and mirrored to the multiple disk drives in the system. One basic principle associated with RAIDs is that the operation of multiple, distinct disk drives in the array must be coordinated. Unfortunately, because of the complexity and large number of disk drives required in RAID systems, such technology has been adopted only by high-end enterprise servers. Because of cost and complexity considerations, data storage systems used by mid-range and small enterprises are typically limited to single or a double-backup configuration, running at standard data rates that are less than 500 Mb/s.
Peak data rates are often referred to in the drive and drive interface literature as a performance metric, including Ultra-320 SCSI, double rate Fibre Channel, etc. These data rates extend up to 2.5 Gb/s, and are achieved by buffering the I/O to the drive. In other words, the buffers enable the data rate to be temporarily higher between the buffer and the computer or other information device that accesses the disk drive than the maximum sustained data rate that can be achieved by the disk drive. Thus, from the standpoint of the computer or other information device that accesses the disk drive, the apparent peak data rate is greater than the actual data rate associated with the disk drive. The use of buffers is most effective at enhancing write data rates, since data that is written to a disk can be easily cached. Buffering is much less effective for reading, as the data to be read is not often in the buffer, and must be found and read using normal accessing techniques, which have normal seek and latency times.
The relatively small increases during recent years in the maximum sustained data rate for disk drives represents a significant limiting factor in the speed at which computers can operate when performing I/O operations that use disk drives. Moreover, because of the complexity and cost of RAID systems, this problem is more pronounced for users of individual personal computers and networks associated with small and medium sized enterprises.